This invention relates to the casting of steel strip by a twin roll caster. In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontally positioned casting rolls, which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product delivered downwardly from the nip. The term “nip” between the casting rolls is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the casting pool will generally be at a temperature in excess of 1550° C., and usually 1600° C. and greater. It is necessary to achieve very rapid cooling of the molten steel over the casting surfaces of the rolls in order to form solidified shells in the short period of exposure on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. Moreover, it is important to achieve even solidification so as to avoid distortion of the solidifying shells which come together at the nip to form the steel strip. Distortion of the shells can lead to surface defects known as “crocodile skin surface roughness.” Crocodile skin surface roughness is known to occur with high carbon levels above 0.065% and even with carbon levels below 0.065% by weight carbon. Crocodile skin roughness involves periodic rises and falls in the strip surface of 40 to 80 microns, in periods of 5 to 10 millimeters, measured by profilometer.
We have found that with carbon levels below about 0.065% by weight the formation of crocodile skin surface roughness is directly related to the heat flux between the molten metal and the surface of the casting rolls, and that the formation of quality cast strip can be controlled by controlling the heat flux between the molten metal and the surface of the casting rolls. As build-up of oxides on the surface of the casting rolls affect the heat transfer through the surface, we have also found that by controlling the energy exerted by rotating brushes peripherally in contact with the casting surfaces of each casting roll, heat flux between the molten metal and the surface of the casting rolls, and in turn quality of the thin cast strip.
This relationship between the heat flux from the molten metal and the surface of the casting rolls and the casting of quality thin cast strip has been found to occur whether the casting roll surfaces are smooth or textured. We have also found that the texture of the casting surfaces of the casting rolls changes during casting with oxide build-up. This change can cause a change in heat flux from the molten metal to the casting roll surfaces and in turn changes the quality of the thin cast strip. As disclosed and claimed in our U.S. application Ser. No. 11/302,485 and published as US 2006-0237162, by controlling the heat flux between the molten metal and the casting roll surfaces, high fluctuations in the heat flux should be avoided during the formation of the metal shells during casting, and in turn control the formation of crocodile skin surface roughness in the thin cast strip produced.
We have now found that by measuring the temperatures of casting surfaces along the length of the casting rolls, across the width of the cast strip adjacent the nip, or both, that interfacial heat transfer coefficient between the molten metal in the casting pool and the casting surfaces of the casting rolls can be monitored and controlled. The measured temperatures are then used to regulate gas delivery within different segments along the surface of the casting roll adjacent the contact of the cleaning brushes. In addition, the energy of the rotating brush against the casting roll also may be controlled at the same time based on the casting speed by varying the application pressure, the angle of contact with the casting roll surfaces, the speed of rotation, or a combination thereof, with an electric, pneumatic or hydraulic motor rotating the brush against the casting surface. The heat flux may also be measured in combination by any available method to improve the quality of cast strip produced.
We have now found that during the casting operation, the temperature of the casting roll at the casting surface will vary along the length indicating a build-up of oxides and corresponding variation in heat flux along the length of the casting roll. Such heat flux variation along the length of the roll, in turn, can cause variations in surface quality across the cast strip width as the strip exits the casting rolls. The variation in temperature and heat flux along the length of a casting roll also causes a temperature variation across the width of the cast strip. Thus, by measuring the temperature variation along a casting roll or across the cast strip, the heat flux variation across the roll and in turn strip quality can be directly monitored, and by utilizing such measured temperature to control localized delivery of gas in segments along the casting rolls adjacent the cleaning brushes, localized control of heat flux can be achieved and quality of cast strip can be improved.
A method of continuous casting of thin cast strip is disclosed that comprises the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip in a casting area with a protective atmosphere;
assembling a rotatable cleaning brush peripherally to contact the casting surface of each casting roll in contact with the casting surfaces between the nip and the casting area;
counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip;
removing oxides from the casting surface of each casting roll by contacting the casting surface of each casting roll with the rotating cleaning brush;
determining temperatures in segments along the length of at least one of the casting rolls, of the cast strip in segments laterally across the strip adjacent the nip, or both; and
delivering gas at the casting surface adjacent the rotating brush varied in segments corresponding to the determined temperatures to provide variable gas delivery in the segments along each casting roll surface adjacent the brushes.
The step of delivering gas at the casting surface may provided by varying gas composition, gas mixture, gas pressure, or a combination thereof, delivered through nozzles in at least three segments extending along the casting surfaces of the casting rolls. In some embodiments there may be five or more segments, and in some embodiments, each gas nozzle may comprise a segment along the surface of the casting roll. In addition, the step of providing gas delivery adjacent the casting roll surface may comprise delivering the gas at the nip formed between the cleaning brush and the casting roll surface, or through a housing adjacent the rotating cleaning brush.
The step of determining temperatures may include measuring temperatures across the casting roll surface at locations in each segment, and then using the measured temperature to control gas delivery across the casting roll surface adjacent the cleaning brushes in each segment. The temperature may be determined adjacent the casting roll nip, in advance of contact of the casting roll surfaces with the cleaning brushes, or after contact of cleaning brushes with the casting roll surfaces and before the casting roll surface again rotates into the casting area where a protective atmosphere is maintained above the casting pool. Alternately or in addition, the step of determining temperatures may involve measuring temperatures of the cast strip in the segments across its width adjacent the nip, and typically between about 0.2 meter and 1 meter from the roll nip.
In any event, the temperatures may be continuously determined along the surface of the casting rolls or across the cast strip adjacent the nip in segments or in a continuum, and directly used to regulate the gas delivery in segments to the casting surfaces adjacent the cleaning brushes, to improve the quality of the cast strip produced. In some embodiments, the temperature across the casting roll or the cast strip, or both, may be determined with sufficient accuracy by location, e.g. by scanning pyrometer, to individually vary the gas delivered to the casting surfaces by each gas delivery nozzle, such that each gas delivery nozzle can be provided as a segment along the surface of the casting roll.
The gas in the gas delivery step may comprise at least one gas selected from the group consisting of nitrogen, argon, helium, hydrogen, water vapor, carbon monoxide, carbon dioxide, dry air or a mixture of two or more thereof.
The method may be used to control the heat transfer coefficient between the casting pool and the casting surface of the casting rolls. The method may further be used to measure the temperature in segments along the casting surfaces and/or the cast strip adjacent the nip to control the energy exerted by the brushes on the casting roll surfaces, and/or the angle of contact of the brushes with the casting roll surfaces, as explained below.
Alternatively, and in addition, disclosed is a method of continuous casting of thin cast strip comprising the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip to provide a casting area with a protective atmosphere;
assembling a rotating cleaning brush peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting area;
counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip;
removing oxides from the casting surface of each casting roll by contacting the casting surface of each casting roll with the rotating cleaning brush; and
determining temperatures in segments of at least one casting roll along the length of the casting roll, of the cast strip along the width as the strip adjacent the nip, or a combination thereof.
The latter method may be used by the operator of the caster to monitor and to identify conditions in the continuous casting where corrective steps should be taken in casting to provide a desired quality of the thin strip being cast.
Further, an apparatus for continuously casting thin cast strip is disclosed that comprises:
a pair of counter-rotating casting rolls spaced laterally to form a nip between circumferential casting surfaces of the rolls through which thin cast strip may be discharged downwardly and a casting area above the nip where a casting pool may be formed with a protective atmosphere there above;
rotating cleaning brushes capable of removing oxides from the casting surfaces of each casting roll, positioned to remove such oxides from the casting surfaces in an area between the nip and the casting area;
at least one temperature sensor capable of providing sensor signals corresponding to the temperatures along at least one casting roll in at least three segments, of the cast strip across the cast strip in at least three segments, or both;
gas nozzles capable of delivering gas adjacent the casting surface of each casting roll between a cleaning brush and the casting area in at least three regulated segments corresponding to the segments where temperature is determined; and
a controller capable of receiving the sensor signals and regulating the delivery of gas through at least one nozzle to each segment corresponding to temperatures sensed in each segment.
The gas may comprise at least one gas selected from the group consisting of nitrogen, argon, helium, hydrogen, water vapor, carbon monoxide, carbon dioxide, dry air or a mixture of two or more thereof. The apparatus may include a housing provided adjacent the brush, and wherein the gas nozzles deliver the gas through the housing. The delivery of gas to each segment may be variable in composition, mixture, pressure, or a combination thereof and may be in at least five segments extending along the casting surfaces of the casting rolls. Alternatively or in addition, the temperature sensor may be capable of providing sensor signals corresponding to the temperatures of the cast strip in segments or in a continuum adjacent the nip, and typically between about 0.2 meter and 1 meter from the casting roll nip.